Generally, power supplies for display apparatus such as cathode ray tube (CRT) display monitor apparatuses commonly used with computer systems arc designed to provide the necessary voltages and currents within the desired specifications using internal control. As for a video display apparatus, typically, the power supply system thereof is provided with a power-off circuit for interrupting the electrical power supply to the display apparatus during a power off state.
In a video display apparatus having audio circuitry, video power savings for video circuitry are achieved by display power management signaling (DPMS) control recommended by video electronics standard association (VESA). It is well known that the DPMS power supply modes may be classified into a normal mode, a standby mode, a suspend mode and a power-off mode. Power management in a computer is described in, for example, U.S. Pat. No. 5,586,333 to Chun-Geun Choi, et al., entitled, Method And Control Apparatus For General Power Management Signal Of Computer Peripheral Equipment In A Compute System, U.S. Pat. No. 5,428,790 to Leroy D. Harper, et al., entitled, Computer Power Management System, and U.S. Pat. No. 5,396,635 to Henry T. Fung, entitled, Power Conservation Apparatus Having Multiple Power Reduction Levels Dependent Upon The Activity Of the Computer System.
The DPMS power supply mode of the display apparatus is decided by the horizontal and vertical synchronizing signals and video signals supplied from a host (i.e., personal computer) supporting the video power savings. If all the synchronizing and video signals are supplied from the host, the display apparatus is operated in the normal power supply mode. When the vertical synchronizing signal only is fed from the host, the power supply mode of the apparatus becomes the standby mode, in which various circuits are put into a reduced power state. When only the horizontal synchronizing signal is supplied from the host, the display apparatus is put into the suspend mode. Then, the power to the audio circuitry is shut off so that the audio circuitry cannot be operated. When none of the synchronizing and video signals are provided from the host, the display apparatus is put into the power-off mode. In such a case, the power to the audio circuitry is also shut off.
However, if the video display apparatus is operated in power saving modes, particularly in the suspend and power-off modes, there may arise a drawback that the power supplied to the audio circuit is shut off without regard to user's will. Consequently, the operation of the audio circuit depends on the power supply modes of the display apparatus.